Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed pre-ignition combustion events. The early combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. Late burn combustion events wherein the combustion is later than intended can also lead to pre-ignition combustion events. Specifically, the late combustion can lead to high exhaust manifold pressures and temperatures, as well as higher than intended exhaust residuals, which raises the probability of pre-ignition events.
The Applicants herein have recognized that in turbocharged engines, late burning cylinder combustion events can raise pressures in the exhaust manifold significantly. During some conditions, the elevated exhaust manifold pressures generated in a late burning cylinder can overcome the exhaust valve spring pressure and potentially open exhaust valves on adjacent cylinders. The resulting filling of a neighboring cylinder with hot exhaust residuals can lead to a pre-ignition event on the adjacent cylinder. The problem can be exacerbated in small volume exhaust manifolds that are specifically designed to reduce turbo lag in turbocharger boosted engines.
Thus in one example, some of the above issues may be at least partly addressed by a method for an engine comprising, in response to a sensed block vibration in a window during an open exhaust valve of a first cylinder undergoing a late combustion event and after exhaust valve closing of a second cylinder, performing a pre-ignition mitigating action in the second cylinder. In this way, late burn induced pre-ignition events can be better detected and appropriately mitigated.
In one example, a first cylinder may be under a late combustion event with spark timing retarded from maximum brake torque (MBT) to provide transient torque control. A controller may then assess the output of one or more knock sensors coupled to an engine block in a (first) window that is adjusted to be during an open exhaust valve event of the first cylinder. The window may be adjusted to be after exhaust valve closing and after intake valve opening of a second cylinder that could receive exhaust residuals from the first cylinder but before intake valve closing and before a spark ignition event in the second cylinder. The sensor output may be filtered in the window. For example, the sensor output may be filtered through a first band-pass filter to filter out a first range of frequencies. In response to the filtered sensor output in the first window being larger than a threshold, it may be determined that an exhaust valve of the second cylinder has been forced open due to elevated exhaust manifold pressures, and that the sensor output was indicative of the cylinder exhaust valve slamming upon return to the exhaust seat. Herein, the high exhaust manifold pressure is due to the late combustion event in the recently firing first cylinder that generates a large amount of hot exhaust residuals and the exhaust valve slamming represents the (unintended) delivery of the hot residuals into the neighboring cylinder via forced opening of the exhaust valve. Based on the identity of the recently firing cylinder, the identity of engine cylinders that have their exhaust valves at a base circle of an exhaust camshaft, and a timing of the knock sensor output, the second cylinder receiving the hot residuals from the late burn event may be identified. An engine controller may then perform a pre-ignition mitigating action in the affected cylinder. For example, the controller may disable fuel injection or enrich fuel injection to the second cylinder receiving the hot residuals so as to reduce a temperature of the hot residuals, in situ, and thereby reduce a likelihood of late-burn induced pre-ignition.
It will be appreciated that the controller may also use the same knock sensors to identify cylinder knock and pre-ignition events in other cylinders. For example, by filtering the sensor output through a second, different band-pass filter to filter out a second, different range of frequencies, and by assessing the filtered sensor output in a second, earlier window, the controller may determine if knock or pre-ignition has occurred in a third cylinder firing immediately after the first cylinder.
In this way, vibrations detected during an open exhaust valve of a late combusting cylinder can be used to detect elevated exhaust manifold pressures and forced entry of residuals into a neighboring cylinder. By performing a pre-ignition mitigating action in the affected cylinder, a temperature of the received charge can be rapidly cooled to reduce abnormal cylinder combustion events. By improving detection of unintended exhaust valve opening and receipt of hot residuals, mitigating actions can be performed in a timely manner and engine degradation due to late burn induced pre-ignition events can be reduced. In addition, the sensed vibrations can be used to detect cylinder knock and pre-ignition events in firing cylinders. By using the same knock sensors to detect and distinguish cylinder knock, pre-ignition, and forced exhaust residual entry, component and cost reduction benefits are achieved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.